It is known that devices such as cranes or the like are currently used to lift, move and deposit heavy loads within a specific work area.
These devices are provided with lifting units, which use flexible elements such as cables or chains.
In the nautical field, for example, watercraft are moved in shipyards and at port shores usually by using cranes substantially constituted by a wheeled frame which has a C-shaped plan and by a plurality of motorized lifting units, which are arranged so as to face each other in pairs along the lateral wings of the frame and support the opposite ends of a sling for supporting the watercraft to be moved.
Each lifting unit comprises two tackles arranged in series, each constituted by two blocks, a fixed one and a movable one, with one or more pulleys, around which a flexible element winds, which has a first end that is fixed to a motorized winch and a second end, arranged opposite the first end, which is rigidly coupled to one of the blocks of the tackle that is arranged furthest from the winch.
A load grip element, such as a hook, a sling or others, depending on the type of application for which the lifting unit is intended, is fixed to each movable block.
The elevation of the two movable blocks is independent, so as to allow the lifting unit to adapt to the contour of the load to be moved and balance the load on the various grip elements.
These known lifting units are not free from drawbacks, which are observed in particular in the movement of the movable blocks when empty.
In this step, in order to lift the movable block of the tackle that lies furthest from the winch, which due to friction always moves after the tackle that lies closest to said winch, it is necessary to move the movable block of said tackle to the end of its stroke, with an obvious waste of time on the part of the assigned operators.
Moreover, if the movable blocks of the two tackles are stopped at different levels, due to an uneven distribution of the weight of the flexible elements on said blocks, the lowest block may fall until the other one reaches the end of its stroke, with the risk of injuring operators and/or damaging the objects that are in the work area.
A first known solution for obviating these drawbacks is to provide two winches, on each of which a respective end of the flexible element is wound.
In the portion of the flexible element that connects the two tackles there is a device for temporarily locking the sliding motion, which is activated in order to allow the unladen movements of the two movable blocks to be independent, each movement being actuated by a respective winch.
Even this solution is not free from drawbacks, including the fact that it increases considerably the cost and complexity of the lifting unit and penalizes its operating flexibility and efficiency, in view of the high energy overall energy absorption.
As an alternative, it is known to provide lifting units with mechanical devices that are suitable to temporarily rigidly couple the pulleys of the fixed block of the tackle that lies closest to the winch, simulating the presence of a single loop of flexible element around said first tackle, so as to increase the useful driving torque on the winch and allow the movement of the movable block of the second tackle without having to first move the block of the first tackle to the end of its stroke.
In this case also, there is a device for temporarily locking the temporary sliding of the flexible element arranged along the portion that connects the two tackles, so as to isolate them from each other.
However, this solution requires frequent maintenance and does not ensure optimum and constant performance over time, also in terms of workplace safety.